The present invention relates to a target having a satisfactorily controllable composition, with which a thin metal oxide film having a proper composition can be efficiently produced by laser deposition method, process for producing the target, and process for forming the thin metal oxide film by using the target.
One of the most general methods for producing a metal oxide is a solid-phase method, in which solid-phase raw materials such as carbonate and oxide are mixed so as to give a desired metal oxide composition rate and then sintered to obtain a metal oxide. In the method, however, the obtained metal oxide obviously has a heterogeneous phase microscopically, since solid phases of the respective materials are mixed with each other to be sintered. In particular, in case of synthesizing a multi-element oxide which requires starting materials having a highly homogeneity, it is inevitable that the obtained metal oxide composition shift undesirably due to generation of a different phase. In order to obtain a homogeneous starting material, it is necessary to be synthesized through a homogeneous system from a starting raw material. As such a method, known have been liquid-phase methods based on chemical processes such as a sol-gel method and a coprecipitation method.
According to these conventional liquid-phase methods, however, it is not easy to avoid the essential problem of the heterogeneous system. The reason is as follows. Even if the starting solution is homogeneous, each kind of metal compound therein has its own hydrolysis rate and solubility product etc. The various hydrolysis rates and solubility products inevitably make this system heterogeneous in the following steps of, for example, hydrolysis, neutralization and precipitation, although the obtained powder included in the system is a fine powder. To solve the above problem, proposed has been a method including forming a metal complex between a metal ion and polycarboxylic acid, e.g., citric acid, in an aqueous phase, and then adding polyhydric alcohol, e.g., ethylene glycol, as a crosslinking agent for an ester polymerization to obtain a gel complex polymer.
In this method, however, a part of the metal may separate from the metal complex in the ester polymerization step and this may result in the segregation. In addition, in case of using the obtained complex polymer as a starting material, the product obtained by sintering the gel complex polymer needs to be pulverized finely. It is pointed out that the pulverization step may results in problems such as raising the manufacturing cost and complicating the work procedure.
Besides these, various methods for synthesizing a starting material have been proposed. But any of the methods is not suitable for versatile use because of the high manufacturing cost and complicated operation.
A method for synthesizing ceramics from metal chelate complexes has been developed recently. This method is remarkable because of its possibility of suppressing the undesirable metal composition shift. In this method, however, an appropriate means for mixing solid-phase metal chelate complexes homogeneously at a molecular level has not been found. Thus, the method does not sufficiently make use of the advantage of the metal chelate complexes.
In addition, in any of the conventional methods for producing a metal oxide powder that are practically used currently, it is almost impossible to control, for example, the particle shape and particle size of the obtained powder.
On the other hand, as a method for producing a thin film of multi-element metal oxide, chemical vapor deposition (CVD) and physical vapor deposition (PVD) have been known. In CVD, a raw material is caused to react in a gaseous-phase state and the objective compound is separated out in a solid-phase state. In PVD, a solid raw material (target) is vaporized by the injection of physical energy to rearrange the vaporized material into a thin oxide film on a substrate. Of these, CVD has a drawback that a different phase is likely to be separated during the intermediate reaction due to various vapor pressures of respective kinds of raw materials. For this reason, when CVD is applied to the method of producing a multi-element metal oxide, it is considered to be difficult to control the composition of the obtained metal oxide. On the other hand, sputtering, which is a typical PVD process, is for versatile use, while it has a drawback that the compositional control is considerably difficult to be made in case of using a compound target. This is because the use of an identical compound target for a long period of time may change the composition of the target.
In order to improve the problem of the compositional control, developed has been a laser deposition method (laser abrasion) in which a target is clustered by a laser irradiation to rearrange the compounds contained in the target on a substrate. However, strong laser energy has been needed for clustering the conventional targets, since they have strong bonds such as ionic and covalent bonds. Additionally, large agglomerated molecules (droplet) of the target material may drop on the substrate, resulting in a problem that a heterogeneous thin film (i.e., a thin film having a rough surface) is obtained.
In order to solve the problem, it has been tried to provide a cloison between the target and the substrate to suppress the deposit of such a droplet on the substrate. Using the cloison, however, is only a physical means and it cannot be said a fundamental solution. In addition, the obtained thin film according to PVD is originally no better than a thin film in which the compounds in the target are rearranged. Thus, the thin oxide film quality depends on the target quality. It is therefore necessary to prepare a high-quality target. But, it is difficult to synthesize a high-quality homogeneous bulk of multi-element metal oxide to be used as the target. The synthesis also needs a lot of time and effort, and thereby the synthesis step may be the rate-determining step, resulting in a disadvantageously high cost.
The present invention is intended to overcome the aforementioned conventional problems. It is therefore an object of the invention to provide a target having homogeneity at a molecular level in any component system. The target can be used for a speedily, low-energy and low-cost production of a high-quality thin metal oxide film, especially thin film of multi-element metal oxide, having a proper-controlled composition and a satisfactory surface smoothness. It is another object of the invention to provide a method for producing the above-mentioned target easily. A further object of the invention is to provide a method for effectively forming the above-mentioned thin metal oxide film having a proper-controlled composition with using the target.
According to the present invention, the target for forming a thin metal oxide film, which can achieve the aforementioned objects, is distinctively obtained by molding an amorphous powder of organic metal chelate complexes into the shape of a tablet. The preferable method for producing such a target is as follows. Metallic materials are mixed with an organic chelating agent so as to give a predetermined metal composition, to prepare a transparent aqueous solution of organic metal chelate complexes. The aqueous solution is then spray-dried to obtain an amorphous powder of the organic metal chelate complexes in which the complexes are mixed with each other at a molecular level, followed by the press molding to shape into a tablet.
As the organic chelating agent in this case, preferably used may be an amino-carboxylic acid chelating agent which is not thermally decomposed at 200xc2x0 C. or less. In addition, the chelating agent with at least stoichiometric quantity of a total of respective metallic material is preferably mixed with the metallic materials so as to allow all of the metallic materials for forming complexes completely, to obtain a transparent aqueous solution. Moreover, when the metallic materials contain the metal which is easy to be oxidized by air to be a metal oxide or a higher-valence metal ion, a reducing agent and/or an antioxidant may be added to the aqueous solution of the organic metal chelate complexes to prevent oxidation of the metal ions in the solution. For example, when the metallic materials contain titanium, a reducing agent may be effectively added to stabilize titanium (III).
When the laser deposition method is applied to the production of a thin film by the use of the above-mentioned target, a composition of the obtained thin metal oxide film is easily controllable in any composition system. And also, by such a method, a thin metal oxide film having a satisfactory surface smoothness without a defect can be obtained speedily with lower energy and lower cost.
Accordingly, this method is, in a high degree of efficiency, applicable to the formation of a high-temperature superconductive thin film of multi-element metal oxide such as thin films of YBa2Cu3O7-xcex4 and SrTiO3.